Second, the winding paths of this speaker membrane put torque and axial load on the single 3mm OD ball bearing (with 1mm balls inside!) that I had been using to apply the wire. After a few jobs, this roller would have the characteristic "crunch" of an overloaded bearing. I owe an offering to St. Venant. To fix this, I made a small roller applicator, held by two 3mm OD bearings. This effectively limits the torque seen by the bearings and doubles the effective radial load (because there are two instead of one). It also allows me to experiment with different applicator profile shapes. Two are shown above, on a penny for scale.

The two bearings definitely held up better than the single bearing, but I still managed to overload them eventually. This leads me to think that failure mode is a result of either contamination in the unsealed bearings, or significant axial force during plotting. I think both might be solved by switching to a plain bearing instead of a ball bearing. Plain bearings can be made to be much more compact and to take much more force than ball bearings. The trade-off is generally a higher coefficient of friction as compared to properly working ball bearings. I'm going to make a plain bearing version of the applicator (either with porous oil-embedded SAE 841 bronze, or simply with a lubricated brass bearing) to see if the friction is at an acceptable level for plotting the wire.

The two bearings definitely held up better than the single bearing, but I still managed to overload them eventually. This leads me to think that failure mode is a result of either contamination in the unsealed bearings, or significant axial force during plotting. I think both might be solved by switching to a plain bearing instead of a ball bearing. Plain bearings can be made to be much more compact and to take much more force than ball bearings. The trade-off is generally a higher coefficient of friction as compared to properly working ball bearings.

I'm making a plain bearing version of the applicator with porous oil-embedded SAE 841 bronze to see if the friction is at an acceptable level for plotting the wire. I reamed the bore with a brand new .081" ream in hopes of not destroying the porous structure at the interface which provides the bearing lubrication.